Method for extra-high speed transformation of time signal into two-dimensional space signal

ABSTRACT

A signal light pulse to be converted into a two-dimensional space signal and a reference ultra-short light pulse are directed to a dispersion device, a second-harmonic is generated by introducing a one-dimensional frequency light distribution obtained by a one-dimensional Fourier transform lens, the second-harmonic is then subjected to time-to-space conversion through a one-dimensional Fourier transform lens so as to obtain a light wave distribution, and the light wave distribution is then subjected to filtering by a time-frequency filter provided on a filter plane of a one-dimensional space frequency filtering optical system and is further converted into a two-dimensional space signal corresponding to a time-frequency expanded two-dimensional light distribution which represents a relation between time and frequency of the signal pulse light. Thereby, there is provided a specific technique of realizing ultra-fast conversion of signal form from time signal into two-dimensional space signal without any active-scan and also displaying the two-dimensional signal in a visible region with a temporally steady state.

TECHNICAL FIELD

The present invention relates to a method for ultra-fast conversion oftwo-dimensional information encoded to an ultra-short light pulse signalinto a two-dimensional space signal.

BACKGROUND ART

In optical communication, transmission capacity has been increased byusing methods such as time multiplexing or wavelength multiplexing forrealization of real-time transmission of multi-media information(particularly, image information). The signal form used for thereal-time transmission is basically a time signal. Because of thetransmission capacity increase, it is necessary to perform conversion(encoding) of information to be transmitted into time signals andexpansion (decoding) of the information converted into the time signalsat a ultra-high speed. For this reason, there has been proposed a“space-to-time-to-space signal processing method” which realizesultra-fast conversion of signal form between time signal and two or moredimensional space signal such as image information. However, thisspace-to-time-to-space signal processing method has a problem that mustbe solved in time-to-two-dimensional space signal conversion technology.

Various methods have been proposed as the method for realizingultra-fast conversion of signal form between time signal and spacesignal. However, although the conventional methods are capable ofcarrying out ultra-fast conversion into one-dimensional space signal,some of them basically need to perform active scanning for conversioninto two or more dimensional space signal and thus have a limitation intransformation speed. On the other hand, there are some methods that donot need to perform active scanning, but space signal converted by suchmethods cannot be observed with any ultra-fast light-receiving deviceavailable today.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodfor ultra-fast conversion of time signal into two-dimensional spacesignal which is capable of realizing ultra-fast conversion of signalform from time signal to two-dimensional space signal without performingactive-scan and also is capable of displaying the two-dimensional spacesignal in a visible region with a temporally steady state.

To attain the above-described object, firstly, the present inventionprovides a method for ultra-fast conversion of time signal intotwo-dimensional space signal, wherein a signal light pulse and areference ultra-short light pulse having an appropriate width in spaceare introduced into a nonlinear crystal through a dispersion device anda one-dimensional Fourier transformation optical system. Asecond-harmonic which is generated by satisfying phase matchingcondition in the nonlinear crystal in subjected to time-to-spaceconversion through an inverse one-dimensional Fourier transformationoptical system so as to be converted into a one-dimensional spacedistribution, the time-to-space converted one-dimensional spacedistribution is subjected to filtering with a time-frequency filterprovided on a filter plane of a one-dimensional space frequencyfiltering optical system, and a time-frequency expanded two-dimensionallight distribution representing a relation between time and frequency ofthe signal pulse light is regarded as a two dimensional space signal.

Secondly, the present invention provides a method for ultra-fastconversion of time signal into two-dimensional space signal, wherein asignal light pulse and a reference ultra-short light pulse having anappropriate width in space are introduced into a dispersion device atangles symmetric with respect to the optical axis. Light waves from thesignal light pulse and the reference ultra-short light pulse which are adispersed due to a time difference generated by a difference of incidentpositions on the dispersion device are passed through a one-dimensionalFourier transformation optical system so as to be converted intoone-dimensional frequency light distributions having different incidentangles depending on the incident positions on the dispersion device. Theone-dimensional frequency light distributions is introduced into anonlinear optical crystal, a second-harmonic which is generated bysatisfying phase matching condition determined depending on an angleformed by the incident one-dimensional frequency light distributions issubjected to time-to-space conversion through an inverse one-dimensionalFourier transformation optical system so as to be converted to anone-dimensional space distribution, the time-to-space convertedone-dimensional space distribution is converted into a one-dimensionalspace frequency distribution by a one-dimensional Fourier transformationoptical system, and the one-dimensional space frequency distribution issubjected to filtering by a time-space filter. The light wave thusobtained is subjected to time-frequency expansion through an inverseone-dimensional Fourier transform optical system so as to obtain anintensity distribution of a two-dimensional light distribution, and thetime-frequency expanded two-dimensional light distribution representinga relation between time and frequency of the signal pulse light isregarded as a two dimensional space signal.

Thirdly, the present invention provides the above-described methods forultra-fast conversion of time signal into two-dimensional space signal,wherein space frequency filtering is employed as the time-frequencyfilter, and fourthly, the present invention provides either of theabove-described methods for ultra-fast conversion of time signal intotwo-dimensional space signal, wherein the time-frequency filter has adifferent transmissivity distribution and a vertical cut out position ofa space frequency component of a light wave outputted from theone-dimensional Fourier Transform light system is arbitrarily selected.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of the structure oftime-to-two-dimensional space signal conversion optical system to beused in the method for ultra-fast conversion of time signal intotwo-dimensional space signal according to the present invention.

Reference numerals designate the followings.

-   1: time-to-two-dimensional space signal conversion optical system-   2: diffraction grating-   3: signal light-   4: reference light-   5: cylindrical lens-   6: nonlinear crystal-   7: one-dimensional frequency distribution of signal light-   8: one-dimensional frequency distribution of reference light-   9: cylindrical lens-   10: cylindrical lens-   11: time-frequency filter-   12: cylindrical lens-   13: two-dimensional space distribution-   100: incident plane-   101: nonlinear crystal plane-   102: time-to-space conversion plane-   103: time-frequency filter plane-   104: output plane

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention has the forgoing features, and the embodimentthereof will be described hereinafter.

FIG. 1 shows an example of the structure of the time-to-two-dimensionalspace signal conversion optical system for performing the method ofultra-fast conversion from time signal to two-dimensional space signalaccording to the present invention. This time-to-two-dimensional spacesignal conversion optical system 1 is capable of converting a signallight pulse being a time signal, which is ultra-short pulse laser lightin this example, into a two-dimensional space signal corresponding totime and frequency by using a dispersion device such as a diffractiongrating, a one-dimensional Fourier transformation lens, aone-dimensional inverse-Fourier transformation lens, a nonlinear crystalfor generation of a second-harmonic, a one-dimensional space frequencyfiltering system, and time-frequency filter. That is, as shown in PLO1,signal lights (3) and (4) are introduced into a diffraction grating (2),which is a dispersion device, at angles symmetric with respect to theoptical axis, and thereby light waves are deflected in a direction basedon the diffraction formula. Because each of the signal light and thereference light has a certain width in its incident beam, a timedifference occurs depending on their incident positions onto thediffraction grating. Then, the light waves are subjected to Fouriertransform with respect to horizontal components by a one-dimensionalFourier transform optical system composed of a cylindrical lens (5), andthereby spectrum distributions of the signal light and the referencelight are obtained on a nonlinear crystal plane (101) as spacedistributions. Because the propagation direction (wave vector) of thelightwave differs depending on a difference of the incident positiononto the diffraction grating, the wavefront of the light wave rotateswith time in the nonlinear crystal plane 101.

When the light waves of the signal light and the reference light areintroduced into the nonlinear crystal, there in emitted asecond-harmonic whose wave vector is a sum of two wave vectors by phasematching due to nonlinear effect. Although the wave vectors of the lightwaves of the signal light and the reference light rotate with time, thesecond-harmonic continues to be emitted in a steady state on time basisbecause change rates of the rotation with time are equal. Therefore, thewave vector of the second-harmonic depends on only initial wave vectorsof the wave vectors of the two light waves, so that it depends on arelative time between the signal light and the reference light. Further,within the nonlinear crystal, the light wave of the signal light isscanned by the light wave of the reference light and thus the spacedistribution of the second harmonic becomes equal to the spectrumdistribution 7 of the signal light.

The second-harmonic thus obtained is image-formed on a time-frequencyfilter plane (103) by an image-formation optical system comprised ofcylindrical lenses (9)(10). As a time-frequency filter (11), a filterdesigned such that the frequency of frequency component to be cut out isincreased in order in a vertical direction is used. By changingtransmissivity distribution of the time-frequency filter, it is possibleto set up which frequency component can be cut out to which position inthe vertical direction.

The second-harmonic filtered by the time-frequency filter has a wavevector corresponding to time in a horizontal direction and adistribution corresponding to frequency in a vertical direction. Thissecond-harmonic is subjected to Fourier transformation about itshorizontal direction component by a one-dimensional inverse Fouriertransformation optical system comprised of a cylindrical lens (12). As aresult, there is obtained, on an output plane (104), a two-dimensionalspace distribution (13) of the light wave-having a time distribution inthe horizontal axis direction and a spectrum distribution in thevertical direction. Consequently, it is possible to convert the timesignal contained in the ultra-short light pulse into the two-dimensionalspace distribution of time and frequency.

Of course, the present invention is not restricted to theabove-described example and may be changed or modified in various ways.For example, although the diffraction grating is employed as thedispersion device in the above-described example, another dispersiondevice may be used. Further, although the cylindrical lenses areemployed as the Fourier transformation optical system and theinverse-Fourier transformation optical system in the above-describedexample, another optical device may be used. Furthermore, although thetransmission type filter is used as the time-frequency filter in theabove-described example, a phase type filter may also be used.

INDUSTRIAL APPLICABILITY

As described above, according to the method for ultra-fast conversion oftime signal into two-dimensional space signal of the present invention,it is possible to convert the time signal into its correspondingtwo-dimensional space signal at an ultra-high speed without performingactive-scan unlike the conventional method, and also display theconverted signal directly with a visible light when an infrared rayfalling in a wavelength region used in optical transmission is employed.

1. A method for ultra-fast conversion of time signal intotwo-dimensional space signal, the method comprising: introducing asignal light pulse and a reference ultra-short light pulse having anappropriate width in space into a nonlinear crystal through a dispersiondevice and a one-dimensional Fourier transformation optical system;subjecting a second-harmonic, which is generated by satisfying a phasematching condition in the nonlinear crystal, to time-to-space conversionthrough an inverse one-dimensional Fourier transformation optical systemso as to be converted into a one-dimensional space distribution; andsubjecting the time-to-space converted one-dimensional spacedistribution to filtering with a time-frequency filter provided on afilter plane of a one-dimensional space frequency filtering opticalsystem, wherein a time-frequency expanded two-dimensional lightdistribution representing a relation between time and frequency of thesignal light pulse is regarded as a two-dimensional space signal.
 2. Amethod for ultra-fast conversion of time signal into two-dimensionalspace signal, the method comprising: introducing a signal light pulseand a reference ultra-short light pulse having an appropriate width inspace into a dispersion device at angles symmetric with respect to anoptical axis; passing light waves from the signal light pulse and thereference ultra-short light pulse, which are dispersed due to a timedifference generated by a difference of incident positions on thedispersion device, through a one-dimensional Fourier transformationoptical system so as to be converted into one-dimensional frequencylight distributions having different incident angles depending on theincident positions on the dispersion device; introducing theone-dimensional frequency light distributions into a nonlinear opticalcrystal; subjecting a second-harmonic, which is generated by satisfyinga phase matching condition determined depending on an angle formed bythe incident one dimensional frequency light distributions, totime-to-space conversion through an inverse one-dimensional Fouriertransformation optical system so as to be converted to a one-dimensionalspace distribution; converting the time-to-space convertedone-dimensional space distribution into a one-dimensional spacefrequency distribution by a one-dimensional Fourier transformationoptical system; subjecting the one-dimensional space frequencydistribution to filtering by a time-frequency filter; and subjecting thelight wave thus obtained to time-frequency expansion through an inverseone-dimensional Fourier transform optical system so as to obtain anintensity distribution of a two-dimensional light distribution, whereinthe time-frequency expanded two-dimensional light distributionrepresenting a relation between time and frequency of the signal lightpulse is regarded as a two-dimensional space signal.
 3. The method forultra-fast conversion of time signal into two-dimensional space signalaccording to claim 1, wherein a space frequency filter is employed asthe time-frequency filter.
 4. The method for ultra-fast conversion oftime signal into two-dimensional space signal according to claim 1,wherein the time-frequency filter has a different transmissivitydistribution and a vertical cut out position of a space frequencycomponent of a light wave outputted from the one-dimensional FourierTransform light system is arbitrarily selected.
 5. The method forultra-fast conversion of time signal into two-dimensional space signalaccording to claim 2, wherein a space frequency filter is employed asthe time-frequency filter.
 6. The method for ultra-fast conversion oftime signal into two-dimensional space signal according to claim 2,wherein the time-frequency filter has a different transmissivitydistribution and a vertical cut out position of a space frequencycomponent of a light wave outputted from the one-dimensional FourierTransform light system is arbitrarily selected.
 7. The method forultra-fast conversion of time signal into two-dimensional space signalaccording to claim 3, wherein the time-frequency filter has a differenttransmissivity distribution and a vertical cut out position of a spacefrequency component of a light wave outputted from the one-dimensionalFourier Transform light system is arbitrarily selected.